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Saturn’s moon Enceladus surprisingly comet-like

By Stephen Battersby

Heat radiates from the entire length of 150-kilometre-long fractures on the south pole of Saturn’s icy moon Enceladus

(Image: NASA/JPL/GSFC/SwRI/SSI)

Saturn’s curious moon Enceladus appears to have the same chemical makeup as a comet, according to the latest results from the Cassini probe. That’s a big surprise, as Enceladus should have formed in very different conditions from those of comets.

As well as water vapour, the INMS detected carbon dioxide, methane and a range of more complex organic chemicals such as propane.

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“The organics are clearly there in abundance beyond what we expected,” says INMS lead scientist Hunter Waite of the Southwest Research Institute in San Antonio, Texas, US. “And the composition is very like the composition of a comet.”

“This is very exciting,” says Cassini scientist Julie Castillo of NASA’s Jet Propulsion Laboratory in Pasadena, California, US. “It indicates that Enceladus and comets were made of the same initial materials, and/or affected by similar internal processes,” she told New Scientist.

That is rather puzzling because comets are thought to have formed far from the Sun, out in the region of Uranus and Neptune, says INMS co-investigator Roger Yelle of the University of Arizona in Tucson, US. Enceladus, on the other hand, is thought to have grown within the “Saturnian subnebula” – the cloud of gas that coalesced into Saturn and its major moons.

“The temperature and pressure should have been very different, so you should get different gases,” Yelle told New Scientist.

Liquid water

Enceladus is almost certainly not a captured giant comet, but cometary stuff might have been incorporated into the moon. Ices from the outer solar system might have infiltrated the Saturnian subnebula, suggests William McKinnon of Washington University in St Louis, Missouri, US.

Or comets might have hit Enceladus during a period of upheaval in the solar system around 4 billion years ago called the late heavy bombardment.

During the flyby, Cassini’s infrared camera mapped the heat emissions of the south pole more clearly than before, showing that a great quantity of heat is coming out along the four fractures called “tiger stripes”. Temperatures along these stripes are higher than their surroundings by up to 90 °Celsius.

Life’s ingredients

“The closer we look, the higher the temperatures,” says John Spencer of the Southwest Research Institute in Boulder, Colorado, US, who works on the infrared detector. “It is entirely possible that there is liquid water below the surface of these fractures.”

“We see on Enceladus the three basic requirements for the origin of life,” says Larry Esposito of the University of Colorado in Boulder, principal investigator of another Cassini instrument, the Ultraviolet Imaging Spectrograph (UVIS). “There is water – although it may not be liquid – plus organics and heat.”

Cassini will revisit Saturn’s comet impersonator in August and October, when it might fly even closer to the moon.

Cassini&colon; Mission to Saturn – Learn more in our continually updated special report.